More than 200,000 total hip replacement (THR) surgeries are performed in the U.S. each year. Such surgery is needed when the cartilage cushioning of the joint deteriorates (osteoarthritis), causing pain and disability. The statistical survivorship of THR devices declines dramatically after ten years of use, and few reach 15 years of useful life because the sliding interface between the femoral head (usually CoCr) and the acetabular cup/liner (usually Ultra High Molecular Weight Polyethylene (UHMWPE), or cross-linked polyethylene (XLPE)) breaks down or wears excessively. This lack of durability results from wear of the prosthetic device, which has unacceptable effects, such as riskier revision surgery or surgery postponement with its attendant pain and disability. Typical prosthetic devices have smooth bearing surfaces. In fact, the current engineering paradigm for combating implant wear is to manufacture smoother bearing surfaces.
Several types of bearings for THR devices exist: ceramic-on-ceramic (COC), ceramic-on-metal, ceramic-on-polyethylene, metal-on-metal, and metal-on-polyethylene (MOP). Femoral heads are thus typically either made of metal or ceramic material. Metal heads, often made of cobalt-chromium for hardness and fatigue resistance, are machined to size and then polished to reduce wear of the socket liner. Ceramic heads are typically smoother than polished metal heads, which enables them to operate in the hydrodynamic lubrication regime, thus reducing wear. Despite this advantage, COC hips are prone to edge-loading wear that leads to squeaking, problems that for instance MOP bearings do not exhibit. To date, the potential of also creating fluid film lubrication in MOP devices has been neglected, likely because the polymer bearing is compliant, prone to dimensional error, and not feasible to polish.
Particles generated by adhesive wear, corrosive wear, and/or abrasive wear can also act as abrasives and accelerate deterioration of the sliding interface. Additionally, adverse immunological reaction to indigestible microscopic wear debris can lead to osteolysis, which can undermine the implant and cause instability, which is a leading cause of revision surgery. Thus, preventing or significantly reducing formation of wear particles in the sliding interface between the acetabular component and the femoral head is an ongoing challenge.